Exclusive selectivity of multidentate ligands independent on the oxidation state of cobalt: Influence of steric hindrance on dioxygen binding and phenoxazinone synthase activity

Article abstract of DOI:10.1039/c3dt53546k

The present report describes the syntheses, characterizations, crystal structures and study of the phenoxazinone synthase activity of two peroxo-bridged dicobalt(iii) complexes, [Co₂(L¹) ₂(μ-O₂)](ClO₄)₄·2CH ₃CN (1) and [Co₂(L²)₂(μ-O ₂)](ClO₄)₄ (2), and three mononuclear cobalt(ii) complexes, [Co(L³)(CH₃CN)](ClO ₄)₂ (3), [Co(L⁴)(H₂O)](ClO ₄)₂ (4) and [Co(L⁵)(H₂O)](ClO ₄)₂ (5), derived from the pentadentate ligands L ¹-L⁵, which are the 1:2 condensation products of triamines and 2-acetylpyridine or 2-pyridinecarboxaldehyde (6-methyl-2- pyridinecarboxaldehyde). X-ray crystallography reveals exclusive selectivity of the acyclic Schiff dibasic form of the ligands over the heterocyclic analogues, and this selectivity is found to be insensitive to the oxidation state of cobalt. Other first row transition metals have been characterized in either form of the ligands in their complexes but it is specific for cobalt established in the present study. The pronounced effect of the methyl substitutions is observed from their crystal structures; substitution at imine-C does not have any significant influence on the peroxo-bridging but substitution at sixth position of pyridyl ring prevents the formation of peroxo-bridging, and both the steric and electronic factors play vital roles on such chemical diversity. All the complexes show the phenoxazinone synthase mimicking activity and the comparative catalytic activity has been explored. Although electrochemical behaviors of all the complexes are very similar, their relative catalytic activity mimicking the function of phenoxazinone synthase arises from the electronic and steric factors of the methyl substitution. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53546k

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PAPER
Exclusive selectivity of multidentate ligands
independent on the oxidation state of cobalt:
influence of steric hindrance on dioxygen binding
and phenoxazinone synthase activity†
Cite this: Dalton Trans., 2014, 43,
7760
Anangamohan Panja
The present report describes the syntheses, characterizations, crystal structures and study of the phenox-
1
azinone synthase activity of two peroxo-bridged dicobalt(III) complexes, [Co
2
(L )
2
2 4 4 3
(μ-O )](ClO ) ·2CH CN
2
3
(
(
1) and [Co
2
(L )
2
(μ-O
2
)](ClO
4
)
4
(2), and three mononuclear cobalt(II) complexes, [Co(L )(CH
3 4 2
CN)](ClO )
4
5
1
5
3), [Co(L )(H
2
O)](ClO
4
)
2
(4) and [Co(L )(H
2
O)](ClO
4
)
2
(5), derived from the pentadentate ligands L –L ,
which are the 1 : 2 condensation products of triamines and 2-acetylpyridine or 2-pyridinecarboxaldehyde
6-methyl-2-pyridinecarboxaldehyde). X-ray crystallography reveals exclusive selectivity of the acyclic
(
Schiff dibasic form of the ligands over the heterocyclic analogues, and this selectivity is found to be insen-
sitive to the oxidation state of cobalt. Other first row transition metals have been characterized in either
form of the ligands in their complexes but it is specific for cobalt established in the present study. The
pronounced effect of the methyl substitutions is observed from their crystal structures; substitution at
imine-C does not have any significant influence on the peroxo-bridging but substitution at sixth position
of pyridyl ring prevents the formation of peroxo-bridging, and both the steric and electronic factors play
vital roles on such chemical diversity. All the complexes show the phenoxazinone synthase mimicking
activity and the comparative catalytic activity has been explored. Although electrochemical behaviors of
all the complexes are very similar, their relative catalytic activity mimicking the function of phenoxazinone
synthase arises from the electronic and steric factors of the methyl substitution.
Received 18th December 2013,
Accepted 18th February 2014
DOI: 10.1039/c3dt53546k
www.rsc.org/dalton
(
OAPH) to the phenoxazinone chromophore in the final step
Introduction
6
for the biosynthesis of actinomycin D. The later is one of the
Dioxygen is known as ultimate “green” oxidant as it is atom most potent antineoplastic agents known which inhibits DNA-
economic and freely available in the environment. Transition- dependent RNA synthesis by intercalation of the phenoxazi-
7
,8
metal active sites of proteins play vital roles in dioxygen none chromophore to DNA. Only a few functional models
binding and oxidation of a variety of important biological pro- based on the transition-metal ions have been reported which
cesses because they facilitate the spin–forbidden interaction show phenoxazinone synthase activity, and none of the reports
1
–3
between dioxygen and organic matter.
Study of the model dealt with structure–property correlation towards the phenoxa-
9
–16
complexes of such metalloenzymes is therefore of great impor- zinone synthase mimicking activity.
Therefore, the straight-
tance as those guides for the development of small molecule forward structure–property correlation for the phenoxazinone
catalysts for specific oxidation reactions of organic substances synthase is still lacking, indicating further that the problem of
in the industrial and synthetic processes. One of the major modeling the phenoxazinone synthase activity remains still
4
,5
multicopper oxidase enzyme is phenoxazinone synthase,
which is found naturally in the bacterium Streptomyces antibio-
open.
A number of transition metal complexes with multidentate
ticus. The function of this enzyme is to catalyze the oxidative ligands derived from the condensation reaction of aromatic
coupling of two molecules of a substituted o-aminophenol aldehydes with α,ω-di-primary polyamines have been exten-
1
7,18
sively studied.
Condensation reactions of such aldehydes
with triamine that contains both the primary and secondary
amines not only produce Schiff dibases, but also Schiff mono-
base with an additional saturated heterocyclic ring (imidazoli-
Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS,
Purba Medinipur, West Bengal 721 152, India. E-mail: ampanja@yahoo.co.in
†
Electronic supplementary information (ESI) available: Fig. S1–S9, Scheme S1
1
9
dine or hexahydropyrimidine).
Moreover, metal assisted
and Table S1. CCDC 959180–959184 for complexes 1–5. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3dt53546k
isomerization among these products through formation or
7760 | Dalton Trans., 2014, 43, 7760–7770
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2
0,21
opening of the heterocyclic ring is quite common.
Boča
2
2
et al. have suggested that the Lewis acidity of the metal ion
plays important role as it competes with the acidity of the
δ−
–
CvN– site, so that there is a delicate balance between N
δ+
atom and C center coming together (ring-closing of hetero-
cycle) and going apart (ring-opening of heterocycle). The direc-
tion in which the system will shift depends on the relative
acidity of imine-C and metal centers. Weak Lewis acids such
2
+
2+
2+
as Fe , Co , and Cu are incapable (or only partly capable) of
opening the heterocyclic ring, while strong Lewis acids (e.g.,
3
+
2+
Fe and Zn ) can readily do so with the gain of the denticity
1
9
of ligands. On the other hand, Tuchagues et al. pointed out
that size of the metal ion plays deciding role in shifting the
ligand equilibrium towards the tautomeric form most appro-
priate to each specific cation. Low-spin Fe has a small
enough ionic radius (75 pm) to stabilize the imine isomer in
its complex, while the ionic radius of Ni (83 pm) is probably
too large for allowing coordination of the imine tautomer, and
thus it stabilizes the cyclic analogue.
2
+
2
+
We have recently reported the study of the oxidation of
o-aminophenol (OAPH) by some mononuclear cobalt(III) and a
bis(imido)-bridged dinuclear cobalt(III) complexes in aerobic
2
3–25
condition.
These results suggested that the electronic Scheme 1 Drawing of the ligands characterized in the present study.
environment around the metal centers along with the labile
position in metal sites plays significant role for such activity.
Our continuing interest in the bioinspired catalytic chemistry
thus lead us to explore the reaction between 2-pyridinecarbox-
aldehyde (6-methyl-2-pyridinecarboxaldehyde) and various
triamines in a 2 : 1 molar ratio those are known to afford a
mixture of both the Schiff dibases and their cyclic analogues.
Indeed, such ligands including several functions namely
amine, imine, pyridine and imidazolidine (hexahydropyrimi-
ability of selective dioxygen complexation but also shows the
catalytic activity similar to the phenoxazinone synthase.
Experimental section
Materials and physical measurements
dine) are able to provide nitrogen donors allowing tuning of Chemicals such as 2-pyridinecarboxaldehyde, 6-methyl-2-pyri-
the ligand field around the metal center, and thereby, may dinecarboxaldehyde, 2-acetylpyridine, diethylenetriamine,
lead to significant improvement of the catalytic activity. Fur- 3,3′-diamino-N-methyldipropylamine, 3,3′-bisaminopropylamine,
thermore, cobalt(II) complexes with nitrogen rich ligands are cobalt(II) perchlorate hexahydrate (Aldrich) and o-aminophenol
found to produce μ-peroxo bridged dinuclear complexes by the (Merck, India) were of reagent grade, and used without further
transfer of electrons from the metal centers to the oxygen purification. Solvents like methanol, diethyl ether, and aceto-
molecule. I envisioned that condensation product of aromatic nitrile (Merck, India) were of reagent grade and used as
aldehyde (ketone) and various triamines may at least partly received.
activate molecular dioxygen, thereby, capable to show various
Microanalyses for C, H and N were carried out using a
oxidase activity. Keeping all these in mind, herein, I report the Perkin-Elmer 240 elemental analyzer. The infrared spectra
syntheses and characterizations of two peroxo-bridged of the complexes in KBr pellets were recorded in the
1
−1
dicobalt(III) complexes, [Co (L ) (μ-O )](ClO ) ·2CH CN (1) and 400–4000 cm range on a PerkinElmer Infrared spectrometer.
2
2
2
4 4
3
2
[
Co
2
(L )
2
(μ-O
2 4 4
)](ClO ) (2), and three mononuclear cobalt(II) Electrochemical data were collected on an EG&G Princeton
3
4
complexes, [Co(L )(CH
3
CN)](ClO
4 2 2 4 2
) (3), [Co(L )(H O)](ClO ) (4) Applied Research potentiostat model 263A with a Pt working
5
1
5
and [Co(L )(H O)](ClO ) (5), where L –L are pentadentate electrode, Pt auxiliary electrode and Ag/AgCl reference elec-
2
4 2
ligands as shown in Scheme 1. The preferential complexation trode. Absorption spectra were measured using a UV-2450
ability of all the ligands to the cobalt center irrespective of its spectrophotometer (Shimadzu) with a 1-cm-path-length quartz
oxidation state, and in one case (3) methanol addition across cell. Room temperature magnetic susceptibility measurements
the –CvN– bond, are analyzed on the basis of the obtained were performed in an EG and G PAR-155 vibrating sample
4
structural data. The phenoxazinone synthase-like activity and magnetometer, using Hg[Co(SCN) ] as a reference material;
its detailed kinetic investigations for all the complexes have diamagnetic correction was made by Pascal’s constants for all
also been described. This study not only provides ample infor- the constituent atoms. Electrospray ionization mass spec-
mation regarding the role of the metal ions and other factors trometry (ESI-MS positive) was performed using a Micromass
on the relative stability of condensation products and the Q-tof-Micro Quadruple mass spectrophotometer.
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4
Synthesis of the complexes
[Co(L )(H
2
O)](ClO
4
)
2
(4). A mixture of 3,3′-bisaminopropyl-
amine (0.262 g, 2.0 mmol) and 6-methyl-2-pyridinecarboxalde-
Caution! Perchlorate salts of metal complexes are potentially hyde (0.488 g, 4.0 mmol) was heated to reflux in a 30 ml of
explosive and should be handled in small quantities and with methanol for 45 min resulting in a yellow color solution and it
great care.
was allowed to cool at room temperature. To the resulting
CN (1). Reaction of 3,3′-bisami- yellow solution a 20 ml methanol solution of Co(ClO ·6H
1
[Co
2
(L )
2
(μ-O
2
)](ClO
4
)
4
·2CH
3
4
)
2
2
O
nopropylamine (0.262 g, 2.0 mmol) with 2-pyridinecarboxalde- (0.730 g, 2 mmol) was added and the mixture was stirred for
hyde (0.428 g, 4.0 mmol) in a 30 ml of methanol under reflux 20 min. Orange crystalline materials containing single crystals
for 45 min afforded a pale-yellow solution. Addition of Co- for X-ray diffraction were separated out from the solution upon
4 2 2
(ClO ) ·6H O (0.730 g, 2.0 mmol) dissolved in 20 ml of metha- standing at room temperature overnight (1.128 g, 92% yield).
nol to the resulting solution at room temperature instantly pro- Anal. Calcd for C H Cl CoN O : C, 39.15; H, 4.73; N, 11.41.
2
0
29
2
5 9
duced a light brown solution. The mixture was stirred in air Found: C, 39.07; H, 4.89; N, 11.16. IR (KBr, cm− ): 3428 br
1
for 30 min during which time a light brown powders separated (νOH); 3220 br (νNH); 2854–3050 w (νCH), 1654 m (νCvN);
out from the solution. It was collected by filtration and washed 1600 m (ν
); 1120 s, 1083 s, 625 s (νCl–O). UV-vis λmax/nm
CvN, py
−1
with mother liquor followed by methanol/ether and finally air (ε/dm mol cm−1) in methanol: 276 (23 878), 332 sh (1481).
3
dried. Yield: 1.020 g (82%). Single crystals suitable for X-ray ESI-MS (positive) in methanol: Base peak was detected at m/z =
4
+
diffraction were obtained from slow evaporation of acetonitrile 495.11 corresponding to [Co(L )(ClO )] .
4
5
solution of the complex after one week. Anal. Calcd for
Co 18: C, 38.48; H, 4.19; N, 13.46. Found: C, described for 4 using 3,3′-diamino-N-methyldipropylamine
8.57; H, 4.09; N, 13.36. IR (KBr, cm ): 3235 w (νNH); (0.145 g, 1.0 mmol), 6-methyl-2-pyridinecarboxaldehyde
854–3062 w (νCH); 1630 m (νCvN); 1601 m (νCvN, py); 1145 s, (0.244 g, 2.0 mmol) and Co(ClO ·6H O (0.355 g, 1.0 mmol)
114 s, 1087 s, 636 s, 626 s (νCl–O); 777 w (νO–O). UV-vis λmax yielded 0.502 g (88%) of crystalline materials including single
2 4 2
[Co(L )(H O)](ClO ) (5). An identical methodology as that
C
40
H
52Cl
4
2 12
N O
−1
3
2
1
4
)
2
2
/
3
−1
−1
nm (ε/dm mol cm ) in methanol: 474 (3287), 438 (3264), crystals for X-ray quality of complex 5. Anal. Calcd for
10 (3118), 322 sh (7220), 256 (20 125). ESI-MS (positive) in CoN : C, 40.16; H, 4.94; N, 11.16. Found: C, 40.07;
methanol: Base peak was detected at m/z = 467.09 corres- H, 4.99; N, 11.06. IR (KBr, cm ): 3420 br (νOH/NH); 2839–3062
4
C
21
H
31Cl
2
5 9
O
−
1
1
+
ponding to [Co(L )(ClO )] .
w (νCH); 1654 m (νCvN); 1600 m (ν
); 1107 s/br, 623 s
4
CvN, py
−1
2
2 4 4
(μ-O )](ClO )
(2). An identical synthesis as that of (νCl–O). UV-vis λmax/nm (ε/dm mol cm−1) in methanol: 292
3
[Co
2
(L )
2
1
was performed using 3,3′-bisaminopropylamine (0.262 g, (19 870) 354 sh (1383). ESI-MS (positive) in methanol:
2
.0 mmol), 2-acetylpyridine (0.488, 2.0 mmol) and Co- Base peak was detected at m/z = 509.12 corresponding to
5
+
(ClO
4
)
2
·6H
2
O (0.730 g, 2.0 mmol) as starting materials which [Co(L )(ClO
4
)] .
led to 0.940 g, (77%) of a dark-brown crystalline materials.
Crops of dark-brown crystals suitable for X-ray analysis were
X-ray crystallography
obtained from slow evaporation of acetonitrile solution of the Single crystal X-ray diffraction data for complexes 1–5 were col-
complex at ambient temperature over one week. Anal. Calcd lected on a Bruker SMART APEX-II CCD diffractometer using
for C H Cl Co N O : C, 39.29; H, 4.45; N, 11.45. Found: graphite monochromated Mo Kα radiation (λ = 0.71073 Å).
4
0
54
4
2 10 18
C, 39.47; H, 4.39; N, 11.36. IR (KBr, cm−1): 3248 br Several scans in φ and ω directions were made to increase the
(
νNH); 2856–30 862 w (νCH); 1639 m (νCvN); 1602 m (νCvN, py); number of redundant reflections and were averaged during
1
145 s, 1117 s, 1087 s, 637 s, 626 s (νC–O); 780 m (νO–O). refinement cycles. Unit cell determination, data processing
3
−1
−1
UV-vis
λ
max/nm (ε/dm mol
cm ) in methanol: 401 and structure solution were performed using the Bruker Apex-
(4683), 272 (23 420). ESI-MS (positive) in methanol: II suite program. All available reflections to 2θmax were har-
A major peak was detected at m/z = 495.12 corresponding to vested and corrected for Lorentz and polarization factors with
2
+
26
[
Co(L )(ClO
[
4
)] .
4 2
CN)](ClO )
Bruker SAINT plus.
Absorption corrections, inter-frame
3
Co(L )(CH
3
(3). A similar methodology as scaling, and other systematic errors were performed with
2
6
described for synthesis of 1 was carried out using 2-pyridine- SADABS. All the structures were solved by the direct methods
carboxaldehyde (0.428 g, 4.0 mmol), diethylenetriamine and all non-hydrogen atoms were refined anisotropically by
2
27
(
4 2 2
0.204 g, 2.0 mmol) and Co(ClO ) ·6H O (0.730 g, 2.0 mmol) full-matrix least squares based on F using the SHELXL-97
2
8
as starting materials which yielded 0.730 g, (57%) of a red crys- program within the WINGX package considering all available
talline materials. Dark-red crystals suitable for X-ray diffraction reflections. During the development of the structure it became
were obtained from slow evaporation of acetonitrile solution of apparent that few atoms in 2, namely C9 and O6–O12, were
the complex after several days. Anal. Calcd C H Cl CoN O : disordered. These disordered atoms were allowed to refine
2
1
30
2
6 9
C, 39.39; H, 4.72; N, 13.12. Found: C, 39.37; H, 4.59; N, 12.94. freely, and the final occupancy parameters were rounded
IR (KBr, cm− ): 3244 br (νNH); 2852–3076 w (νCH); 1632 m up and fixed. While refining anisotropically, the Ueq values of
1
(
νCvN); 1599 m (ν
) 1118 s, 1083 s, 622 s, 629 s (νC–O). O6–O8 were larger than usual and thus ADP restraints were
CvN, py
3
−1
−1
UV-vis λmax/nm (ε/dm mol cm ) in methanol: 289 (20 878) applied in order to make them reasonable. The Ueq values of
46 sh (1283). ESI-MS (positive) in methanol: base peak was oxygen atoms of one perchlorate ion in other structures
3
3
+
detected at m/z = 499.10 corresponding to [Co(L )(ClO )] .
(except 5) were also reasonably larger than usual, but no elec-
4
7762 | Dalton Trans., 2014, 43, 7760–7770
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Table 1 Crystal data and structure refinement of complexes 1–5
Compound
1
2
3
4
5
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
40
H
52Cl
4
Co N O
2 12 18
C
40
H
54Cl
4
Co N O
2 10 18
C
21
H
30Cl
2
CoN
O
6 9
C
20
H
29Cl
2
CoN O
5 9
C
21 2 5 9
H31Cl CoN O
1248.60
150(2)
0.71073
Triclinic
Pˉ1
10.5833(5)
10.7937(5)
12.7878(6)
82.279(3)
71.692(3)
64.544(3)
1252.20(10)
1
1222.59
150(2)
0.71073
Monoclinic
C2/c
20.0273(6)
12.7577(5)
21.1058(8)
90
110.357(2)
90
5055.8(3)
4
640.34
150(2)
0.71073
Monoclinic
P2
9.324(4)
7.978(2)
18.741(8)
90
93.935(2)
90
1390.8(9)
2
613.31
150(2)
0.71073
Monoclinic
P2 /a
18.261(1)
8.2488(5)
19.086(1)
90
117.836(3)
90
2542.3(2)
4
627.34
150(2)
0.71073
Monoclinic
P2 /n
8.3498(1)
20.2191(3)
15.6642(3)
90
99.775(1)
90
2606.12(7)
4
1
1
1
γ (°)
Volume (Å )
3
Z
D
calc (g cm−
3
)
1.656
1.606
1.529
1.602
1.594
Absorption coefficient 0.962
0.950
0.868
0.945
0.923
−
1
(mm
)
F(000)
642
2.68–27.44
9526
2520
3.31–26.37
18 412
662
2.98–29.95
8951
1268
3.45–27.51
8013
1300
3.61–27.49
11 684
θ Range ion (°)
Reflections collected
Independent
5644/0.0340
5119/0.0368
6274/0.0545
5136/0.0396
5959/0.0181
reflection/Rint
Data/restraints/
parameters
Goodness-of-fit on F
Final R indices
4361/0/326
3970/1/366
5430/1/340
3484/4/311
5003/0/354
2
1.051
1.045
1.135
1.044
1.025
R
1
= 0.0945,
wR = 0.2130
= 0.1183,
wR = 0.2426
R
1
= 0.0789,
wR = 0.2089
= 0.1005,
wR = 0.2272
R
1
= 0.1080,
wR = 0.2657
= 0.1170,
wR = 0.2755
R
1
= 0.0995,
wR = 0.2549
= 0.1399,
wR = 0.2840
R
1
= 0.0290,
wR = 0.0695
= 0.0393,
wR = 0.0742
[
I > 2σ(I)]
2
2
2
2
2
R indices (all data)
R
1
R
1
R
1
R
1
R
1
2
2
2
2
2
tron density around these atoms was found and therefore only
ADP restraints were applied for these cases. Hydrogen atoms
were mostly included in the structure factor calculation in
Results and discussion
Syntheses and general characterizations
geometrically idealized positions with thermal parameters Condensation of 2-acetylpyridine with triamine exclusively
depending on the parent atoms by using a riding model. Rests results Schiff dibasic ligand because methyl substitution on
of them were located in the difference Fourier maps and were the azomethine-C not only reduces the electrophilic character
isotropically treated based on their respective parent atoms. of the imine-C but also increases the steric hindrance to
Relevant crystallographic data together with refinement details the approaching nucleophile. On the other hand, it is well
for 1–5 are listed in Table 1.
explored that the condensation of a triamine containing both
primary and secondary amine groups with 2-pyridinecarbox-
aldehyde in 1 : 2 molar ratio yields a mixture of Schiff dibase
Kinetics of the phenoxazinone synthase activity
The phenoxazinone synthase mimicking activity of all the five and its heterocyclic analogue, and the ratio of which can be
1
9,22
complexes was studied by the UV-vis spectroscopy in dioxygen established by NMR spectroscopy.
Recent reports disclosed
saturated methanol using o-aminophenol (OAPH) as a sub- that the metal-assisted ring opening/closing processes are
strate at 25 °C. Kinetics of the aerobic oxidation of OAPH cata- common for such ligand systems, and several factors are
lyzed by 1–5 was investigated monitoring growth of the responsible in this event including size of the metal ions,
3
−1
18–22,29
absorbance as a function of time at 435 nm (ε = 24 × 10 M
Lewis acidity, and stereochemistry of the metal centers.
−
1
cm ), which is characteristic of 2-aminophenoxazine-3-one in Keeping all these in mind, various triamines were allowed to
methanol. In order to determine the dependence of the rates react with 2-acetylpyridine or 2-pyridinecarboxaldehyde
on the substrate concentration and various kinetic parameters, (6-methyl-2-pyridinecarboxaldehyde) in 1 : 2 molar ratio at
−
5
−5
1
× 10 or 2 × 10 M solution of the complexes were treated reflux that resulted yellow-orange condensation products.
with at least 10 equivalents of the substrate so as to maintain Treatment of in situ prepared ligands with Co(ClO ·6H O in
the pseudo-first-order condition. Similarly, varying amounts of aerobic condition afforded two peroxo-bridged dicobalt(III)
)
4
2
2
−
5
1
2
the catalyst, (5–600) × 10 M, were treated with a fixed concen- complexes of [Co
2
(L )
)](ClO ) (2), while the rests are mononuclear cobalt(II)
4 4
2
(μ-O
2 4 4 3 2 2
)](ClO ) ·2CH CN (1) and [Co (L ) -
tration (1 × 10− M) of the substrate in dioxygen saturated (μ-O
2
2
3
4
methanol in order to examine the catalyst dependency on rate complexes, [Co(L )(CH
of the reaction. The rate of a reaction was determined by linear and [Co(L )(H O)](ClO )
2 4 2
CN)](ClO
(5). For complexes 1 and 2, the cobalt(II)
regression from the slope of absorbance versus time plot and ions are oxidized by the molecular oxygen which in turn
) (3), [Co(L )(H O)](ClO ) (4)
3
4 2 2 4 2
5
2
−
averaged over three independent measurements.
reduced to the peroxide ion, O
2
, that bridges the cobalt(III)
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centers. Purity of the complexes was established by elemental
analyses; those are consistent with the structural composition
as found from the single-crystal X-ray crystallography. All the
complexes are soluble in common polar organic solvents (e.g.,
acetonitrile, methanol and DMF) as well as in water reflecting
their ionic character.
The IR spectra of complexes 1–4 display moderately strong
−
1
peaks in the range 3220–3248 cm that indicates N–H stretch-
ing vibration, while it overlaps with the OH stretching fre-
quency of the coordinated water molecules in 5. The later is
−
1
eventually observed at 3420 cm as a broad signal that corro-
borate the facts of extended hydrogen boning present in the
solid state of 5. Similar broadening of stretching frequency of
−
1
Fig. 1 Molecular structure of 1 showing C–H⋯π stacking interactions.
Ellipsoids are drown in 30% probability and most of the hydrogen atoms
are omitted for clarity.
coordinated water molecule is found at 3428 cm for complex
. In the IR spectra of complexes 1–5, a strong and sharp
band due to azomethine ν(CvN) appears in the range
4
−
1
1
625–1654 cm . In addition, a sharp peak in between 1597
−
1
and 1601 cm is observed for all the complexes, which might
be assigned to pyridyl CvN stretching. The most important
feature of IR spectra for 1 and 2 is the appearance of a
medium intense band for peroxo O–O stretching at 776 and
88 cm− , respectively, which fall in the same range as that
1
7
reported in structurally characterized end-on bridged dicobalt(III)
3
0,31
complexes.
Moreover, the IR spectra show strong intense
−
1
absorption band over the range of 1083–1120 cm which is
unambiguously assigned to the stretching frequencies of per-
chlorate ions. The splitting or broadening of ν(Cl–O) band may
−
results from the involvement of hydrogen bonding of ClO
4
ions in the crystal lattices. The bending mode of vibration for
−
−1
ClO
4
ions is observed in the range 622–633 cm for all the
complexes and they are also broad as expected.
The electronic spectrum of complex 1 displays two bands
and a shoulder at 474, 256 and 322 nm, respectively. The
broad band at 474 and shoulder at 322 nm are attributed to
Fig. 2 Crystal structure of 2 showing C–H⋯π stacking interactions.
Ellipsoids are drown in 30% probability and most of the hydrogen atoms
are omitted for clarity.
III
32,33
the peroxide π* → Co LMCT band,
while higher energy
band might be due to LLCT transition. However, in the elec-
Table 2 Selected bond lengths (Å) for complexes 1–5
tronic spectrum of 2, only two bands observe at 401 and 272.
III
The former is convincingly assigned to peroxide π* → Co
1
2
3
4
5
LMCT band, later one is due to LLCT transition. Both the
peroxo-bridged complexes are highly stable in solution even Co1–N1
1.949(5)
1.940(5)
1.994(5)
1.904(5)
1.997(5)
1.864(4)
1.439(8)
4.479(1)
1.923(4)
1.951(5)
1.996(4)
1.930(4)
1.980(4)
1.885(4)
1.439(7)
4.529(1)
2.233(6)
2.101(6)
2.161(6)
2.179(6)
2.182(6)
2.168(6)
—
2.189(7)
2.074(7)
2.196(8)
2.067(7)
2.255(6)
2.181(5)
—
2.2731(14)
2.0926(14)
2.2057(14)
2.0848(14)
2.2466(14)
2.1444(13)
—
Co1–N2
they can sustain at very high temperature (Fig. S1 ESI†). Nature
of the electronic spectra for mononuclear cobalt(II) complexes
Co1–N3
Co1–N4
3
2
–5 are similar as those exhibit LLCT band at 289, 276 and Co1–N5
Co1–O1/N6
92 nm, respectively. In addition, a relatively weak broad
shoulder for these three complexes is observed in the range
32–354 nm that signs about the LMCT transition.
O1–O1a
Co1–Co1a
—
—
—
3
Structural studies
are found in Table S1 (ESI†). The complex cations in 1 and 2
CN (1) are formed by a μ-peroxo-bridged Co dimer, and in the mole-
1
III
The dimeric compounds Co
2
(L )
2
(μ-O
2
)](ClO
4
)
4
·2CH
3
2
and [Co (L ) (μ-O )](ClO ) (2), crystallize in the centrosym- cular structure, two metal centers are related to each other
2
2
2
4 4
metric triclinic P ˉ1 and monoclinic C2/c space groups, respect- through an inversion center, which is located at the midpoint
ively, where the asymmetric unit in both the cases corresponds of peroxo bond that bridges two centrosymmetrically related
to half of the molecule. The perspective view of the complex metal centers in an end-on fashion. The peroxo nature of the
cations of 1 and 2 are depicted in Fig. 1 and 2, respectively, dioxygen bridge is confirmed by the O–O distance of 1.439(8)
together with the atomic numbering scheme. Important bond Å in 1 and 1.439(7) Å in 2, and separation of the metal centers
lengths are displayed in Table 2, while selected bond angles are found to be 4.479(1) and 4.529(1) Å in 1 and 2, respectively.
7764 | Dalton Trans., 2014, 43, 7760–7770
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These metal–metal separations are significantly longer than
3
0,32
those doubly bridging systems
but comparable to the
reported complexes in which only peroxo unit bridges two
III
34–36
Co ions by means of an end-on (trans) fashion.
The
charge neutralization of the complexes is ensured by the pres-
ence of four perchlorate counter ions and each of them sits on
a general position in 1, while two perchlorate ions reside on a
special position in the asymmetric unit of 2, one being situ-
ated on a 2-fold axis and the remaining one in an inversion
center.
The geometry of the metal centers in these complexes can
be best described as pseudo-octahedral structure, comprised
1
2
of five nitrogen atoms from pentadentate ligands (L or L )
and a bridging peroxide oxygen atom. The pyridyl rings are
found to cis to each other and approximately perpendicular to
each other (dihedral angle 81.30° for 1 and 75.45° for 2), but
one of them is in trans to peroxo group which is different from
the previously reported structure derived from the reduced
Fig. 3 Molecular structure of 5 showing atom numbering scheme.
Ellipsoids are drawn in 30% probability level and hydrogen atoms are
omitted from clarity.
3
5,36
Schiff base ligands of similar kinds.
tances fall in the range 1.904(5)–1.997(5) for 1 and 1.923(4)–
.996(5) for 2, while Co–O bond lengths are 1.864(4) and 1.885(4) further interacts with the pyridyl ring by means of a π–π
The Co–N bond dis-
1
for 1 and 2, respectively, consistent with the low-spin state of staking interaction (not shown) in 1.
III
Co centers. The low-spin state of the metal ions in 1 and 2 is
The molecular structures of 3–5 are very similar and a rep-
further supported by the fact of diamagnetic characteristics of resentative structure of 5 is displayed in Fig. 3, while rests of
these complexes at room temperature. Pyridyl–N bond length them are shown in Fig. S4 and S5 (ESI†) for 3 and 4, respect-
[
1.997(5) for 1, 1.980(4) for 2] trans to the peroxo group is ively. Important bond lengths are displayed in Table 2, while
somewhat longer than cis disposed pyridyl–N [1.949(5) for 1, selected bond angles are found in Table S1 (ESI†). Complexes
.923(4) for 2], which essentially arises from the stronger trans 4 and 5 crystallize in the monoclinic centrosymmetric space
influence exerted by dioxygen moiety. The considerable distor- groups P2 /a and P2 /n, respectively, while complex 3 crystal-
tion from the ideal octahedral geometry (90°/180°) is reflected lizes in the monoclinic chiral space group P2 which might be
1
1
1
1
3
from the transoid and cisoid angles as can be seen from due to the asymmetric nature of ligand L resulting in the
Table S1 (ESI†). The crystal packing of the complexes is even- chiral tunnels. All these three complex cations adopt a dis-
tually stabilized by the strong electrostatic force of attraction torted-octahedral structure (see Table S1 ESI†) being exclu-
between +4 charges of complex cations and perchlorate ions, sively coordinated by five donor sites of the pentadentate
37
and interestingly they form alternative layers in the crystal ligands in an identical manner as found in 1 and 2, and the
lattice (Fig. S2 and S3 ESI† for 1 and 2, respectively). In coordination sphere is completed by either of water molecule
addition, hydrogen bonds involving lattice acetonitrile and (in 4 and 5) or acetonitrile molecule (in 3). The charge of the
secondary amine in 1, and in between secondary amine and complex cation in all these cases is ensured by the presence of
the crystallographically ordered perchlorate ion in complex 2, two perchlorate ions in the crystal structures. The structures of
are observed (Table 3). That solvate acetonitrile molecule 4 and 5 are very similar but only difference is that the central
amine hydrogen in 5 is being substituted by a methyl group.
However, they are significantly different from 3 in that the
central amine nitrogen is connected by two shorter ethylene
Table 3 Selective hydrogen bonds in 1–5
linkers in 3 instead of propylene linkers for others (Scheme 1);
in addition, one azomethine group in 3 is modified by the
addition of methanol molecule across the –CvN– bond.
D–H⋯A
D–H
H⋯A
D⋯A
<D–H⋯A
1
9,38
1
2
3
N3–H3N⋯N6
N3–H3N⋯O4
N3–H3N⋯O2
N3–H3N⋯O5
N4–H4N⋯O3
N4–H4N⋯O4
O1–H1A⋯O6
O1–H1A⋯O8
O1–H1A⋯O5
O1–H1B⋯O8
0.93
0.91
0.93
0.93
0.93
0.93
0.88
0.88
0.84
0.78
2.29
2.07
2.40
2.31
2.46
2.46
2.46
2.39
2.02
2.05
3.205(8)
2.975(7)
3.170(16)
3.187(11)
3.265(16)
3.323(17)
3.04(2)
3.058(17)
2.834(2)
2.817(2)
166
173
140
158
145
155
124
133
165
171
The Co–N bond distances in all the three complexes are very
similar and they vary in the range 2.101(6)–2.233(6) for 3,
2.067(7)–2.255(6) for 4 and 2.0848(14)–2.2731(14) for 5. The
Co–O distance for coordinated water molecule is found to be
a
c
c
2.181(5) and 2.1444(13) for complexes 4 and 5, respectively.
b
b
4
5
These bond lengths are along with the line of the reported
high-spin cobalt(II) complexes having similar coordination
39
environment.
The room temperature magnetic moment
values [3.97, 4.08 and 4.12 BM for 3, 4 and 5, respectively]
further support the high-spin state of the central cobalt(II) ions
Symmetry code: a = 1/2 − x, −1/2 + y, 1/2 − z; b = −1/2 + x, 1/2 − y, z;
c = −1 + x, y, z; d = 1/2 − x, 1/2 + y, 1/2 − z.
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other cases they are mononuclear and oxidation state of the
metal is also +2. The mechanism of formation of peroxo-
bridged dinuclear complexes is well known in which both the
metal centers donate one electron to molecular dioxygen and
each of the metal ions is oxidized by one unit (Scheme S1
ESI†). This proposition suggests that the closer approach of
two metal centers having unsaturated coordination number
towards the molecular dioxygen along with the electron donat-
ing ability of metal centers play vital roles for such dioxygen
binding ability. Effect of the methyl substitution at azo-
methine-C is already noticed in the crystal structure of 2 where
Fig. 4 One dimensional (1D) hydrogen boded supramolecular chain of
3
propagating along the a axis.
in these complexes. The solid state packing of 5 as shown in increased steric congestion enforces the metal centers to stay
2
3
4
Fig. S6 (ESI†) is stabilized by the hydrogen bonding interaction away from each other. Similar to ligand L , ligands L and L
between perchlorate ion acceptor and coordinated water mole- have also two methyl substitutions but the position of substi-
cule and aromatic C–H donors. But molecular packing in 3 tution is different and that is probably the reason for such
and 4 are very interesting; it forms 1D supramolecular straight chemical diversity. Careful inspection of bond distances
chain resulting from the hydrogen bonding interaction reveals that secondary amine bond distances are longer than
between perchlorate ions and secondary amines in 3 (Table 3, the pyridyl moieties in 1 and 2 (see Table 2), which is consist-
Fig. 4), while wavy supramolecular chain propagating along ent with the state of hybridization of nitrogen atoms. But
the crystallographic a axis results from the hydrogen bonds reverse trend is observed in complexes 3–5, which suggests
involving secondary amine, coordinated water molecules and that the methyl substitution in pyridine ring at the sixth posi-
perchlorate ions, later being acted as a sole hydrogen acceptor tion offering significant steric congestion, and thereby, leads
in 4 (Table 3, Fig. S7 ESI†).
to a weaker coordination ability of pyridyl–N atoms in these
4
7,48
A number of information can be obtained from the X-ray complexes.
Based on the above observations one can con-
crystal structures of the complexes. As shown in Fig. 1 and 2, clude that both the steric and electronic factors are involved in
the end-on peroxo-bridge is to some extent supported by C– the dioxygen binding ability of the complexes. Firstly, the
H⋯π interaction in both the complexes. Although the O–O reduced donation from pyridyl nitrogen to cobalt decreases
bond length in 1 and 2 is identical, but the Co–O–O angle is significantly drifting of electron towards the molecular dioxy-
slightly smaller in 1 [112.4(4)°] than that found in 2 gen. Secondly, the steric congestion offered by the methyl sub-
[
113.0(4)°], and this difference in bond angles results the stituents diminishes the possibility of closer approach to each
longer metal–metal separation in 2 [4.529(1) Å] than that of 1 other which is similar to the biological system where steric
4.479(1) Å]. These observations suggest that methyl substitution hindrance of globin chains in hemoglobin inhibits the peroxo
[
2
at imine-C atoms in L increases some sort of steric congestion bridging thereby preventing the formation of hematin.
around the metal center thereby preventing the closer
It is now well explored that aromatic aldehyde reacts with
approach of the metal centers to each other during the dioxy- triamine having central secondary amine functionality pro-
1
9,22
gen complexation. The longer Co–O distance of 1.885(4) Å in 2 duces both Schiff dibase and its heterocyclic analogue,
than that of 1.864(4) Å in 1 is also consistent with the above and substitution like methyl group at pyridine should not have
fact. Literature reports on similar complexes with reduced much impact on such isomerization. Thus in situ prepared
1
3
4
Schiff bases show that both the pyridyl groups are cis to each Schiff base ligands of L , L and L are expected to stay in equi-
other even also cis to peroxo moiety and in those cases the librium with their heterocyclic analogues. But isolated pro-
central secondary amine is occupied trans to the dioxygen ducts suggest that they are in Schiff dibasic form in all the
3
5,36
ligand.
The authors pointed out that pyridyl group may complexes although one arm of the Schiff base is modified by
compete with the dioxygen moiety in π-back donation if it the addition of methanol across the azomethine moiety in 3.
occupies trans position of the peroxo unit, and thereby, the The yields of these reactions also unambiguously tell the story
systems may not be stable enough. Indeed, this proposition of the ring opening of such condensed heterocyclic forms
was well supported by the fact that in all the structurally during complexation. As mentioned earlier, mainly two factors
1
9,20
characterized peroxo-bridged complexes so far the trans posi- (Lewis acidity and size of the metal ion)
are well explored
tion of the dioxygen moiety is occupied by a σ-donor on the selective binding ability of the ligands and/or ring
4
0–45
1
atom,
et al. where the trans position is occupied by a pyridyl group.
The present work is also further strengthening the fact that a cobalt(III) has smaller ionic radius and also high Lewis acidity.
only exception is the recent report by Fukuzumi opening and closing phenomena. The binding of L in
4
6
complex 1 agrees well with both of these factors as the central
3
4
potentially π-acceptor ligand like pyridine may also stabilize But the complexation of L and L in 3 and 4, respectively,
enough the peroxo bridging in its trans position.
does not support either of the factors. The remark of Tucha-
It is quite interesting to note that although the ligands in gues et al. about the prominent role of the ionic radius over
the present study are very similar but L and L could only able the Lewis acidity (postulation of Boča et al.) also breaks down
to produce peroxo-bridged dicobalt(III) complexes, whereas in in the present finding as the ionic radius of Co (83.5 pm) is
1
9
1
2
II
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II
49
a
even larger than that of Ni (83 pm). It is worth noting that Table 4 Electrochemical data for 1–5 in methanol
Mn, Fe, Ni, Cu and Zn complexes have been structurally
characterized in both the isomeric forms of the ligands,
c
p
a
p
1
8–20
Complex
E
(V)
E
(V)
p
ΔE (V)
but the present results establish that it is the Co that exclu-
sively prefers the Schiff-dibasic form of the ligands even irre-
spective of its oxidation state.
1
2
3
4
5
−0.583
−0.626
−0.382
−0.437
−0.453
—
—
−0.084
−0.075
−0.082
—
—
−0.298
−0.372
−0.371
Electrochemical studies
a
Pt working and Ag/AgCl reference electrodes; supporting electrolyte
tetraethylammonium perchlorate; scan rate 100 mV s−
1
.
Since the function of a redox active metalloenzyme is to cata-
lyze a redox process, thus electrochemical properties are
important factors in determining the catalytic activity of their
model complexes although significant modification of poten-
tials may occurs in a chemical reaction especially if the reac-
tants interact with each other. The electrochemical data of the
complexes have been recorded in methanol containing 0.1 M
tetraethylammonium perchlorate as a supporting electrolyte at
a platinum working electrode and using a Ag/AgCl reference
electrode. The cyclic voltammogram of 3 is shown in Fig. 5
and while for 2 in Fig. S8 (ESI†), and the electrochemical data
for all the complexes are given in Table 4. Electrochemical be-
havior of both peroxo-bridged complexes 1 and 2 are similar in
that they are irreversibly reduced to cobalt(II) from cobalt(III) at
the electrode surface. The irreversible nature may be due to
the significant modification of reduced species in term of loss
of bridging peroxide as similar behavior is also observed at
inert condition. In contrast, the electrochemical responses of
3
–5 are quasi-reversible and these electrochemical responses
are due the cobalt(III)/cobalt(II) couple. The electrochemical
properties of these complexes are very similar to the reported
5
0
results of related complexes.
Fig. 6 Time resolved UV-vis spectral changes for the oxidation of
Phenoxazinone synthase like activity
o-aminophenol (1.0 × 10− mol dm ) catalyzed by 1 (1 × 10 mol dm
2
−3
−5
−3
)
The phenoxazinone synthase mimicking activity of the com-
for up to 2 h of reaction in dioxygen-saturated methanol at 25 °C.
plexes was studied by monitoring the oxidation of o-amino-
phenol (OAPH) spectrophotometrically in dioxygen-saturated
methanol at 25 °C. In order to avoid the autoxidation of the
substrate by air the catalytic activity was examined in the
absence of an added base. Prior going to the detailed kinetic
study, it is important to check the ability of the complexes to
behave as catalyst for the oxidation of o-aminophenol, and for
this purpose 1.0 × 10− or 2.0 × 10 M solutions of the com-
plexes were treated with a 0.01 M solution of o-aminophenol,
and the spectra were recorded in dioxygen saturated methanol
at 25 °C. The time dependent representative spectral changes
for a period of 2 h after the addition of OAPH are shown in
Fig. 6 and 7, for 1 and 4, respectively. The spectral scans reveal
the progressive increase of peak intensity at ca. 435 nm,
characteristic of phenoxazinone chromophore, suggesting cata-
lytic oxidation of OAPH in aerobic condition. It may be noted
here that a blank experiment without catalyst does not show
significant growth of the band at 435 nm in an identical reac-
tion condition. These spectral behaviors conclude that all
the complexes show phenoxazinone synthase like activity in
aerobic condition.
5
−5
Fig. 5 Cyclic voltammogram of 4 in methanol containing tetraethyl-
ammonium perchlorate as the supporting electrolyte at a scan rate
−1
of 100 mV s
.
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Fig. 8 Nonlinear plot of initial rates versus substrate concentration for
the oxidation of o-aminophenol catalyzed by complexes 1, 2 and 4.
Symbols and solid lines represent experimental and simulated profiles,
respectively.
Fig. 7 UV-vis spectral scan for the oxidation of o-aminophenol (1.0 ×
−2
mol dm−3) catalyzed by 4 (2 × 10 mol dm ) for up to 2 h of reac-
−5
−3
1
0
tion in dioxygen-saturated methanol at 25 °C.
Kinetic studies were performed to understand the extent of
−5
the catalytic efficiency. For this purpose, a 1 × 10 M solution
of 1 and 2 or 2 × 10− M solution of 3–5, were treated with at
least 10-fold concentrated substrate solution so as to maintain
the pseudo-first-order condition. All the kinetic experiments
were carried out at a constant temperature of 25 °C, monitored
with a thermostat under aerobic conditions. For a particular
complex–substrate mixture, time scan at the maximum band
of 2-aminophenoxazine-3-one was performed for a period of
5
3
0 min. The initial rate of the reactions versus concentration
of the substrate data show a rate saturation kinetics as can be
seen from Fig. 8. This observation indicates that 2-aminophe-
noxazine-3-one formation proceeds through a relatively stable
intermediate, a complex–substrate adduct, followed by the
redox decomposition of the intermediate at the rate determin-
ing step. This type of saturation rate dependency can easily
recall the Michaelis–Menten model, originally developed for
the enzymatic kinetics, which on linearization gives double
reciprocal Lineweaver–Burk plot to analyze values of various
M
parameters like Vmax, K , and Kcat. The representative observed
and simulated initial rates versus substrate concentration of Fig. 9 Linear Lineweaver–Burk plot for complexes 1, 2 and 4. Symbols
and solid lines represent experimental and simulated profiles,
respectively.
non-linear plot and the Lineweaver–Burk plot for complexes 1,
2
and 4 are shown in Fig. 8 and 9, respectively. Analysis of the
experimental data yielded Michaelis binding constant (K
M
)
and Vmax, and the turnover number (kcat) value is obtained by
dividing the Vmax by the concentration of the complex used,
and these parameters are listed in Table 5. Moreover, for a par- Table 5 Kinetic parameters for phenoxazinone synthase activity of 1–5
ticular substrate concentration, varying the complex concen-
3
(mol dm−3
K )
cat (h−1
Complex
V
max (mol dm− s−1
)
K
M
)
tration, a linear relationship for the initial rates is obtained,
which shows a first-order dependence on the complex concen-
tration (Fig. S9 ESI†).
As can be seen from Table 5, the turnover number (kcat) for
the oxidation of OAPH follows the order 1 > 2 ≫ 5 > 4 > 3. The
8.60 (±0.09) × 10−
6.40 (±0.08) × 10−
1.87 (±0.06) × 10−
8
1.01 (±0.15) × 10
1.47 (±0.16) × 10
1.01 (±0.14) × 10
1.28 (±0.15) × 10
1.32 (±0.17) × 10
−2
−2
−2
−2
−2
30.09
23.04
3.36
6.37
8.28
1
2
3
4
5
8
8
−8
3.54 (±0.06) × 10
4.60 (±0.08) × 10−
8
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higher reactivity of complexes 1 and 2 is most probably due to ing activity and the relative extent of the catalytic activity has
ease of the regeneration of the catalyst by molecular oxygen. been examined. Although electrochemical behaviors of all the
Moreover, the high reactivity of 1 in comparison to 2 probably complexes are very similar, their relative catalytic activity
due to the steric hindrance of the two methyl substitution at mimicking the function of phenoxazinone synthase arises
azomethine-C towards the incoming substrate as well as from the electronic and steric factors of the methyl substi-
during the regeneration of catalyst by the reaction of molecular tution specifically at the position of the substitution.
dioxygen. Effect of the methyl substitution has already been
noticed in the crystal structure of 2 where the metal⋯metal
separation as well as Co–O bond distances is slightly longer
than that found in 1. Although electrochemical behaviors of
all the complexes are very similar but the lower reactivity of
Acknowledgements
The author is grateful to the University Grants Commission,
3
–5 can be explained by the chemical sense where the methyl
India for the financial support of this research (Sanction no.
PSW-173/11-12). Jadavpur University and Indian Association
for the Cultivation of Science, India, are also gratefully
acknowledged for the instrumental facilities.
substitution at sixth position of pyridine moieties decreases
significantly the possibility of oxidation by molecular dioxygen
chemically. The lowest kcat value of 3 possibly due to the struc-
tural rigidity of the ligand although significant flexibility is
gained by the addition of methanol molecule across one of the
–CvN– linker. Moreover, the slightly higher reactivity of com-
pound 5 in comparison to 4 can only be possible if the elec-
tronic features dominate over the steric factor that gained in
the presence of an additional methyl group attached to central
amine nitrogen in 5. Although not conclusive, the solid state
structure of complex 5 is quite informative as it shows the
methyl substitution under consideration occupying at the
opposition site of the approaching substrate molecule. It is
also worth noting that the present compounds especially 1
and 2 show even higher reactivity than our recently reported
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7770 | Dalton Trans., 2014, 43, 7760–7770
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